1. Field of the invention
The present invention relates to an image information recorder, and, more particularly, to an image information recorder capable of reproducing image signals in the form of a visual image which have a resolution density different from that of the recorder.
2. Description of the related art
Signals (image signals) of image information can be reproduced in the form of a visual image by an image information recorder, such as a printer, which repeatedly carries out horizontal scanning (primary scanning) and vertical scanning (secondary scanning) in response to the image signals. The horizontal or primary one of the aforesaid scannings is usually carried out along a so called "scanning" line.
The scanning line can be considered as being formed by a great many small dots, which continuously, or at least substantially continuously, exist on a line and each have a certain information, such as black or white, called a gray level. The image signals as mentioned above are provided dot by dot in response to the gray level of the respective dots.
The distance between two adjacent scanning lines is usually made equal to the diameter of a dot, so that dots are regularly arranged in a matrix (dot matrix). Therefore, any two-dimensional image can be reproduced by the collection of dots in a dot matrix which have the respective gray level according to the image. Such collection of dots will hereinafter be called a dot pattern.
The quality, i.e., the resolution, of an image reproduced on the basis of the image signals is determined by the number of dots per unit length of a scanning line. This number of dots is called a resolution density, which is usually represented by the number of dots per inch of a scanning line (abbreviated as dpi, hereinafter).
Presently, various kinds of office automation equipment, e.g., computers, facsimile machines and so on, are used which are often coupled to image information recorders. In these cases, the resolution density of image signals applied to an image information recorder must be equal to that of the recorder or else the image information carried by image signals can not be reproduced exactly. Therefore, in the case where it is necessary to have devices having the different resolution densities are coupled with each other, transformation of resolution density is required between both devices.
For this purpose, various techniques of resolution density transformation have been developed. Briefly, resolution density transformation involves a dot pattern of a certain resolution density being transformed to a dot pattern of a different resolution density. In other words, when respective co-ordinate systems are applied to the dot matrix of original image information and to that of image information to be reproduced, the position and gray level of every dot in the co-ordinates of the image information to be transformed are determined on the basis of the position and gray level of a dot or dots in the co-ordinates of the original image information.
The gray level of every dot in the transformed coordinates is determined by a calculation in accordance with a predetermined algorithm on the basis of the gray level of a dot or dots on the original co-ordinates. During this calculation, there occur cases where some dots in the original co-ordinates are sometimes omitted in the transformed co-ordinates or undesired dots are sometimes added in the transformed co-ordinates, with the result that blurring or fading appears in the visual image reproduced.
The aforesaid disadvantage is caused for the following reason. The gray level of a dot in the transformed coordinates, as the result of the calculation in accordance with a predetermined algorithm, is of a multi-value. Such a gray level signal of the multi-value is compared with a certain threshold value to be converted into a binary signal suited for reproduction by an image information recorder.
In this case, if the value of the threshold signal is always fixed, there occurs the omission of necessary dots or the addition of undesired dots, as mentioned above, according to the figure of the dot pattern included in an original image information, e.g., an oblique line, a right-angled pattern, and so on. In the case of an oblique line, for example, the edges of a reproduced line are not become smoothed, but may be indented because of the inappropriate selection of the value of a threshold signal. Also, the corner of a right-angled pattern may be rounded.
As one of the techniques for improving the disadvantage mentioned above, there has been proposed a resolution density transformation technique as disclosed in the laid-open Japanese patent application JP-A-62/73865(1987).
According to this prior art technique, the value of a threshold signal is varied in accordance with the gray level distribution in an original dot pattern, which contributes to the calculation of the gray level of a dot in the transformed co-ordinates. For example, when the gray level of dots in the corner portion of a right-angled pattern is determined, a threshold signal is changed to a smaller value than usual. If, therefore, the dot pattern of an original image information is so complicated that the figure as mentioned above is included very much, the threshold value must be changed often as much.
However, variance of the value of a threshold signal so often in accordance with the gray level distribution in an original dot pattern results in complications, and such variance is not practical, especially in an image information having a complicated dot pattern.